This invention relates to a method for causing a growth of an epitaxial layer of one conductivity type free from auto-doping on a semiconductor substrate of which the major surface has thereon a semiconductor region of the one conductivity type and another semiconductor region of the opposite conductivity type arranged side by side in an exposed manner.
A conventional method for manufacturing a semiconductor device such as Field Effect Transistors (FET), Field Controlled Thyristors (FCT), Integrated Circuit (IC) and the like includes the process steps of selectively forming on one main surface of a semiconductor substrate a semiconductor region of the opposite conductivity type to that of the semiconductor substrate and thereafter causing a growth of an epitaxial layer of the same conductivity type as that of the semiconductor substrate, on one major surface of the semiconductor substrate. The method for manufacturing, for example, an FET of the type wherein current vertically flows, includes the steps of selectively forming on one major surface of a semiconductor substrate serving as a drain region of one conductivity type (for example, n-type), a semiconductor region of the opposite conductivity type to that of the drain region serving as a gate region and thereafter causing an epitaxial growth on the drain region of an n-type semiconductor region serving as a source region.
Since in the last-mentioned steps the gate region of said opposite conductivity type is formed in advance of the epitaxial growth of the source region, impurity atoms providing said opposite conductivity type are outdiffused, during the epitaxial growth, from an exposed portion of the gate region into the vapor phase. This causes the occurrence of an auto-doping phenomenon that such impurity atoms are mixed into the source region (epitaxial layer) from the vapor phase. The occurrence of such auto-doping phenomenon causes a reduction in the resistivity of the epitaxial layer, a variation in the resistivity thereof from one place to another, and in an extreme case a failure of the conductivity type of the epitaxial layer to become a desired type. These process steps, therefore, have the drawbacks due to such problems that the resultant semiconductor device fails to have a desired characteristic and that the device fails to be of uniform quality.
A prior art which proposes a means for preventing the occurrence of such auto-doping phenomena is disclosed in U.S. Pat. No. 3,716,422 (issued to D. W. Ing. et al on Feb. 13, 1973). Ing et al suppress the auto-doping phenomena by causing a growth of the epitaxial layer in two different stages. Specifically, a first epitaxial layer is grown on one major entire surface of a semiconductor substrate of one conductivity type and having previously formed thereon partially diffused region having the opposite conductivity type to that of such substrate, and then removal is made of said first epitaxial layer other than that corresponding to the diffused region of said opposite conductivity type. Thereafter, a second epitaxial layer is grown on the exposed surface of the semiconductor substrate and remaining first epitaxial layer, to a desired thickness.
According to Ing et al at the time of growth of the first epitaxial layer auto-doping indeed occurs due to the outdiffusion of impurity atoms from the diffused region of said opposite conductivity type into the vapor phase. Since the first epitaxial layer is selectively removed in such a manner that only the exposed surface of the diffused region of said opposite conductivity type remains to be covered by the first epitaxial layer, the auto-doping attributable to the diffused region of said opposite conductivity type can be prevented at the time of growth of the second epitaxial layer.
However, since in the Ing et al method the second epitaxial layer is grown with part of the first epitaxial layer left on the exposed surface of the diffused region, it is difficult to prevent the auto-doping from such part of the first epitaxial layer, which auto-doping, though occurring only slightly, is a serious matter for a semiconductor device which should be precisely fabricated. Further, since according to the Ing et al method the epitaxial layer is grown in two different steps, the steps for making the semiconductor device becomes unavoidably complicated. Further, it is difficult to have the second epitaxial layer selectively grown. This is disadvantageous for the manufacture of a semiconductor device of the type wherein electrodes, for example, are formed on the second epitaxial layer and on the diffused region, respectively.